RFR: 8369186: HotSpot Style Guide should permit some uses of the C++ Standard Library [v2]

[John R Rose]

On Thu, 9 Oct 2025 03:01:03 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> doc/hotspot-style.md line 1971:
>> 
>>> 1969: 
>>> 1970: * `<system_error>`
>>> 1971: 
>> 
>> To continue the roll of rationale,  for `chrono` etc., here are some suggestions which I am just pulling out of my back pocket:
>> 
>> "HotSpot has its own timing support APIs."
>> "Initializer lists are an advanced C++ API feature which have surprising interactions with other initialization syntaxes in use in HotSpot."
>> "HotSpot uses floating point numbers, which are more portable and have more predictable performance characteristics than rational arithmetic."
>> "HotSpot has its own mechanisms for managing errors."
>
> I'll add some stuff to the PR change.
> 
>> "HotSpot has its own timing support APIs."
> 
> The argument for chrono is that our existing APIs aren't serving us well.
> chrono provides strong type safety. We've had multiple cases of mistakes like
> a double seconds being treated as double millis or vice versa, and other
> similar errors. But it would be a large effort to adopt chrono. And we'd need
> to decide whether to use the predefined clocks or hook up chrono to our
> clocks. It may be that using the predefined clocks is fine, but it's a
> question that needs careful study.
> 
>> "Initializer lists are an advanced C++ API feature which have surprising
>> interactions with other initialization syntaxes in use in HotSpot."
> 
> I think the trailing "in use in HotSpot" can be dropped from that. Mostly I
> think they are fine, but the potential ambiguity between some forms of direct
> initialization and initializer list initialization, and the resolution of that
> ambiguity, is unfortunate.
> 
>> "HotSpot uses floating point numbers ..."
> 
> Note that `<ratio>` is a *compile-time* rational arithmetic package. It's also
> fixed (though parameterized) precision. It's not a general purpose rational
> arithmetic facility. My understanding is that it started life as a detail of
> chrono, and was extracted and promoted to a public facility in the belief that
> it has broader utility.
> 
>> "HotSpot has its own mechanisms for managing errors."
> 
> Well, no, we don't really. Or rather, we've got a plethora of bespoke ad hoc
> mechanisms. There's a bunch of discussion happening around that topic lately.
> I don't think we've coalesced around any particular approach yet. And even
> once we decided on something it's likely to take quite a while for rollout to
> really make use of whatever we settle on.

I love a good nerd sniping! Thanks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27601#discussion_r2421306579